Spectroscopy.
Femtosecond transient absorption spectroscopy was carried out with a
Ti:sapphire-based regenerative amplification system as described (38).
A 400-nm pump beam was obtained by frequency-doubling the output from
the amplifier and attenuated to 300 nJ. Femtosecond time delays up to 5
ns between pump and probe were controlled by a delay line, and
time-gated spectra at 109 delay times were recorded. The polarizations
of pump and probe beams were set at the magic angle (54.7°). The
instrument response function was fit to a Gaussian of 120 fs full-width
at half-maximum (FWHM). The samples were loaded in a flow system of
4-ml volume, including a flow cuvette of 1-mm path length, and flowed
by a peristaltic pump.

Data Analysis.
The transient absorption spectra were globally analyzed in terms of a
kinetic model with sequentially interconverting EADS, i.e., 1 → 2 → 3 →
…. The arrows indicate successive monoexponential decays with
increasing time constants, which can be regarded as the lifetime of
each species. The first EADS corresponds to the time 0 difference
spectrum. The EADS are not necessarily associated with pure molecular
species and often reflect mixtures of molecular species. To further
unravel the pathways for product-state formation, a target analysis was
applied, in which a specific kinetic scheme was tested (see Supporting Text
and Figs. 6 and 7). With this procedure, the SADS of the pure molecular
states are estimated. The global and target analysis procedures have
been extensively reviewed in ref. 39.